Hello. If you choose to not trust a computer, you should see the trust alert every time that you connect your iPhone to the untrusted computer in question. If you haven't already, please try disconnecting your iPhone from your computer and plugging it back in.
If this does not cause the popup to reappear, you can try disconnecting your iPhone from your computer and resetting your privacy settings at Settings > General > Reset > Reset Location & Privacy. This will reset the computers you have chosen to trust. After resetting this, please plug your device back in and the dialog box should appear.
More information about the "Trust This Computer" dialog is available here.